Paraffin wax and paraffins may be obtained by various processes. U.S. Pat. No. 2,692,835 and EP2655565 disclose a method for deriving paraffin wax and paraffins from crude oil. Also, paraffin wax and paraffins may be obtained using the so called Fischer-Tropsch process. An example of such process is disclosed in WO 2002/102941, EP 1 498 469, WO 2004/009739, WO 2013/064539 and in WO2014095814.
The Fischer-Tropsch process can be used for the conversion of synthesis gas into liquid and/or solid hydrocarbons. The synthesis gas may be obtained from hydrocarbonaceous feedstock in a process wherein the feedstock, e.g. natural gas, associated gas and/or coal-bed methane, heavy and/or residual oil fractions, coal, biomass, is converted in a first step into a mixture of hydrogen and carbon monoxide. This mixture is often referred to as synthesis gas or syngas. The synthesis gas is then fed into a reactor where it is converted in one or more steps over a suitable catalyst at elevated temperature and pressure into paraffinic compounds and water in the actual Fischer-Tropsch process. The obtained paraffinic compounds range from methane to high molecular weight molecules. The obtained high molecular weight molecules can comprise up to 200 carbon atoms, or, under particular circumstances, even more carbon atoms. Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi-tubular fixed bed reactors, fluidised bed reactors, such as entrained fluidised bed reactors and fixed fluidised bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebulated bed reactors.
Catalysts used in the Fischer-Tropsch synthesis often comprise a carrier-based support material and one or more metals from Group 8-10 of the Periodic Table of Elements, especially from the cobalt or iron groups, optionally in combination with one or more metal oxides and/or metals as promoters selected from zirconium, titanium, chromium, vanadium and manganese, especially manganese. Such catalysts are known in the art and have been described for example, in the specifications of WO 9700231A and U.S. Pat. No. 4,595,703.
The hydrocarbon product stream obtained after the Fischer-Tropsch synthesis comprises mainly paraffinic compounds ranging from methane to high molecular weight molecules. Of this range of products the lighter part (i.e. methane (C1) to butane (C4)) and the heavier part C40+ are the least desired products of the product stream. For the production of paraffins and waxes the most valued are the hydrocarbons ranging from C5 to C40 (C indicating the carbon chain length). The lighter part of the product stream is normally recovered from the product stream as tail gas and can be reused upstream of the Fischer-Tropsch process (for example in the synthesis gas production).
Typically, the Fischer-Tropsch catalyst deactivate in time, and in order to maintain the productivity, the temperature is increased. Higher operating temperature for an “end of run” (EOR) catalyst results in a lower C5+ selectivity and a lighter wax. On the other side, the freshly started (Start of Run (SOR)) catalyst operation at high C5+ selectivity and a heavy wax. The relation between the operating temperatures of the catalyst and the selectivity is for example described on page 217 of “The Fischer-Tropsch and related synthesis”, H. H. Storch; N. Columbic; R. B. Anderson, John Wiley & Sons, Inc., New York, 1951. With the term “lighter wax” is meant that the heavy wax C40+ fraction has less tailing to very long chains. With the term “heavy wax” is meant a C40+ fraction with tailing to long chain number.
There are several ways known to improve the yield of the paraffins and waxes comprising hydrocarbons ranging from C10 to C40 of the product stream obtained from a Fischer-Tropsch reaction. It is possible to change the catalyst formulation and select a catalyst with an improved yield to this desired part of the product stream. The relation between the catalyst formulation and the improved yield of this catalyst due to the formulation change is for examples described in Applied Catalysis A, 161 (1997), page 59-78. Once the catalyst has been selected the distribution is fixed for a large extent. Moreover, even with the same catalyst a relative small change is possible by varying the concentration of CO, H2 and inert in the gaseous stream towards the reactor. The impact of partial pressures and H2/CO on activity and methane selectivity is for example described in Ind. Eng. Chem. Res. 2005, 44, page 5987-5994 and described on page 330, and on 370-372 of “The Fischer-Tropsch and related synthesis”, H. H. Storch; N. Columbic; R. B. Anderson, John Wiley & Sons, Inc., New York, 1951. Finally it is possible to change the operating temperature of the catalyst. The temperature impact on product distribution is for example described on page 217 of “The Fischer-Tropsch and related synthesis”, H. H. Storch; N. Columbic; R. B. Anderson, John Wiley & Sons, Inc., New York, 1951. There is a continuing desire in the art to improve the Fischer-Tropsch process, especially to tune the product distribution for a given catalyst during its use.